Reduced junction area barrier-based photodetector

ABSTRACT

A photodetector structure having a barrier layer disposed between a pair of like-conductively doped semiconductor layers, the barriers layer having a surface area smaller than the surface area of the upper one of the pair of semiconductor layers. A fill material is disposed between outer peripheral edges of the barrier layer and a region between outer peripheral edges of the first and second layers.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. Provisional application Ser.No. 61/952,197 filed on Mar. 13, 2014, which is incorporated herein byreference.

TECHNICAL FIELD

This disclosure relates generally to photodetectors and moreparticularly to photodetectors having a barrier layer disposed between apair of like-conductively doped semiconductor layers.

BACKGROUND

As is known in the art, one type of infrared detector is an nBn or pBpstructure having a bottom, or collector, semiconductor layer of a firsttype conductivity and an upper contact semiconductor layer of the samefirst type conductivity with a barrier layer sandwiched between theupper and lower semiconductor layers. In the case of the pBp structurethe upper and bottom layers are p-type doped layers and in the cease ofthe nBn structure the upper and bottom layers are n-type doped layers.In one example, the upper and bottom layers are InAsSb, and the middlelayer is AlAsSb. A reverse bias voltage is applied between the upper andlower layers. Thus, in the case of the nBn structure, the bottom layeris, for example, at ground potential and the upper layer is at anegative potential relative to ground potential. In response to infraredradiation, minority carriers, holes, pass between the upper and lowerlayers and through the barrier layer while the barrier layer preventsthe flow of majority carriers. Unfortunately, the barrier layer canproduce unwanted dark current

SUMMARY

In accordance with the present disclosure, a photodetector structure isprovided having a barrier layer disposed between a pair oflike-conductively doped semiconductor layers, the barriers layer havinga surface area smaller than the surface area of the upper one of thepair of semiconductor layers.

In one embodiment, a fill material is disposed between outer peripheraledges of the barrier layer and a region between outer peripheral edgesof the first and second layers.

In one embodiment, a method is provided for forming a photodetectorstructure having a barrier layer disposed between a pair oflike-conductively doped semiconductor layers. The method includesselectively removing outer peripheral portions of the barrier layerwhile leaving outer peripheral portions of the upper and lower layers.

In one embodiment, the selectively removing comprises exposing thestructure to a selective etch, such selective etch selectively removingthe outer peripheral portions of the barrier layer while leavingunetched the outer peripheral portions of the upper and lower layers.

In one embodiment, the selectively removing leaves a gap between theupper and lower layers, such gap having as sidewalls remaining portionsof the barrier layer and including filling the gaps with a fillermaterial.

In one embodiment, the filling comprises using atomic layer deposition.

With such as arrangement a junction with reduced area mitigatesgeneration-recombination and surface current mechanisms that can have adeleterious effect on device performance. Isolating the contact andabsorption regions at the sidewall surface also can negate shuntcurrents. To put it another way, the junction area is diminished whilethe absorption volume is maintained. Backfilling the void maintainspixel integrity. Maintaining the shape or volume of the entire pixel inthis manner retains existing designs and fabrication processes, allowingany scalability inherent to the original process.

The details of one or more embodiments of the disclosure are set forthin the accompanying drawings and the description below. Other features,objects, and advantages of the disclosure will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1A is a cross sectional view of a semiconductor wafer structurehaving a bottom, or collector, semiconductor layer of a first typeconductivity and an upper contact semiconductor layer of the same firsttype conductivity with a barrier layer sandwiched between the upper andlower semiconductor layers at an initial stage in the processingthereof;

FIG. 1B is a cross sectional view of a portion of the structure of FIG.1A after a plurality of infrared photodetector pixel elements are etchedinto the wafer, such cross section being taken along line 1B-1B in FIG.1B;

FIG. 1C is a cross sectional view of an exemplary one of the infraredphotodetector pixel elements of FIG. 1D after outer peripheral portionsof the barrier layer has been selectively removed while leaving outerperipheral portions of the upper and lower layers according to thedisclosure;

FIG. 1D is a cross sectional view of the an exemplary one of theinfrared photodetector pixel elements of FIG. 1C after a gap formedbetween the upper and lower layers after the selective removal, such gaphaving sidewalls remaining, such gap being filled with a filler materialaccording to the disclosure;

FIG. 2A is a top view of the wafer of FIG. 1B;

FIG. 2B is a top view of the exemplary one of the infrared photodetectorpixel elements of FIG. 1C;

FIG. 2C is a top view of the exemplary o as the infrared photodetectorpixel elements of FIG. 1D; and

FIG. 3 is a cross sectional sketch of an exemplary one of thephotodetector pixel elements formed in accordance with FIGS. 1A-1D and2A-2C after passivation and metallization according to the disclosure;

FIGS. 4A-4E show a photo detector pixel element at various stages in thefabrication thereof at various stages in the fabrication thereofaccording to another embodiment of the disclosure.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Referring now to FIG. 1A a wafer 10 is shown having a barrier layer 12disposed, here sandwich, between a pair of like-conductively dopedsemiconductor layers 14, 16. Here, the bottom layer 14 is n-typeconductivity, here for example. InAsSb, absorber layer, and the upperlayer 16 is a contact layer also here, for example, n-type conductivity,InAsSb. Here, the wafer 10 includes an n+-type conductive contact layer18 under the bottom layer 14, as shown. Here, the contact layer 18 isconnected to ground potential and the upper layer 16 is connected to apotential negative with respect to ground potential.

Referring now to FIGS. 1B and 2A, a plurality or array 20 of mesas 22are etched into the upper surface of the wafer 10 down to the contactlayer 18, as shown, using conventional lithographic-etching processing.Here, the delineation etch is based on a tartaric acid, phosphoric acid,and hydrogen peroxide dilution. Each of one of mesas 22 corresponds toone of a plurality of photodetectors in the array 20.

Next, referring to FIGS. 1C and 2B, the structure is subjected to here,for example, a timed, wet etch chemistry of, here for example, asolution of HCl and H₂O₂ to selectively remove edge exposed regions ofthe barrier layer 12 while leaving unetched the layers 14, 16 and 18producing the structure as shown. More particularly, the undercuttingetch is selective to Al containing semiconductors like AlAsSb. Itattacks AlGaAsSb. It should be noted that other wet selective etches oreven a dry selective etch may be used. In any event, the surface area ofthe barrier layer 12 (when viewed from the top of the structure lookingdownward) is reduced. When viewed in cross section as in FIG. 1C, thecross sectional area of the barrier layer 12 is reduced. It should alsobe noted that a gap 21 is, as a result of the selective etching process,formed between the upper and lower layers 14, 16, such gap 21 having assidewalls remaining portions of the barrier layer 12. Next, referring toFIGS. 1D and 2C, the structure is placed in a vacuum chamber, not shown,to deposit, here for example, grow by atomic layer deposition, adielectric or polymer back-fill material, 24, here, for example, silicondioxide.

Next, the structure is passivated with any suitable passivation layer 25and metalized with contacts 27 as Shown on FIG. 3 for an exemplary oneof the photodetectors 22.

Referring now to FIGS. 4A-4E a photodetector pixel element includes inaddition to layers 12 and 18 and barrier layer 14, a pair of n absorberlayers 14 a and 14 b disposed on opposite sides of the barrier layer 18;for example, n absorber layer 14 a absorbs IR radiation having onewavelength and n absorber layer 14 b absorbs long IR radiation at adifferent wavelength.

A number of embodiments of the disclosure have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the disclosure. Forexample, the wafer 10 may be a pBp wafer. Further, the photodetector maybe used in an array, such as a focal plane array. Accordingly, otherembodiments are within the scope of the following claims.

What is claimed:
 1. A photodetector structure comprising: a barrier layer disposed between a pair of like-conductively doped semiconductor layers, the barriers layer having a surface area smaller than the surface area of the upper one of the pair of semiconductor layers.
 2. The photodetector recited in claim 1 including a fill material disposed between outer peripheral edges of the barrier layer and a region between outer peripheral edges of the first and second layers.
 3. A method for forming a photodetector structure having a barrier layer disposed between a pair of like-conductively doped semiconductor layers comprising: selectively removing outer peripheral portions of the barrier layer while leaving outer peripheral portions of the upper and lower layers.
 4. The method recited in claim 3 wherein the selectively removing comprises exposing the structure to a selective etch, such selective etch selectively removing the outer peripheral portions of the barrier layer while leaving unetched the outer peripheral portions of the upper and lower layers.
 5. The method recited in claim 4 wherein the selectively removing leaves a gap between the upper and lower layers, such gap having as sidewalls remaining portions of the hairier layer and including filling the gaps with a filler material.
 6. The method recited in claim 5 wherein the filling comprises using atomic layer deposition. 